if you build a user control and you want to allow design time adding of sub controls to the user control then you need to add a designer attribute to the control definition.
[csharp][Designer("System.Windows.Forms.Design.ParentControlDesigner,System.Design", typeof(System.ComponentModel.Design.IDesigner))]
public partial class MyUserControl : UserControl
{
// control implementation…
}

[/csharp]